1. Field of the Invention
The invention relates in general to wireless communications, and more particularly to discriminating between a periodic interfering signal and a periodic signal of interest.
2. Description of the Related Art
Carrier Sense Multiple Access (CSMA) is a Media Access Control (MAC) protocol in which a communication node verifies the absence of other traffic before transmitting on a shared physical medium, such as a prescribed radio frequency (RF) band. A wireless communication node, for example, may comprise a device with a radio communication card. Carrier sense signifies that a communication node listens for a carrier wave transmitted by another node when trying to send its own transmission. The presence of a carrier on the medium indicates that the medium is busy, i.e. another node currently is transmitting. If a node that intends to transmit information senses a carrier on the medium, then that node waits for the transmission in progress to finish before initiating its own transmission.
CSMA with Collision Avoidance (CSMA/CA) is a protocol in which a communication, node (or station) that intends to transmit sends a jam signal. After waiting a sufficient time for all other nodes that may access the medium to receive the jam signal, the node transmits a data frame. Conversely, before a communication node transmits information onto the shared medium, it listens to determine whether a jam signal has been sent by another node. If it detects a jam signal then it delays its own transmission for a random amount of time before again trying to transmit onto the medium. The random delay causes different nodes to wait different periods of time before again trying to transmit and avoids two or more of them sensing the medium at the same time, finding the channel idle, transmitting simultaneously, and having their transmissions collide with each other.
A jam signal sent by a wireless communication node comprises a signal pattern that informs other nodes that they should postpone transmitting onto the communication medium. In a CSMA/CA network, a transmitting node typically transmits a jam signal as a preamble to a data packet. The sending node sends the jam signal before transmission of the actual data in order to inform other nodes that the sending node intends to transmit data onto the medium. A jam signal alerts other nodes to back off by different random intervals before transmitting data onto the medium. Backing off by different random amounts reduces the probability of a collision when these other nodes first attempt a transmission retry.
Environmental noise is a significant challenge in wireless communications networks. For example, frequencies emitted by a cordless phone, a microwave oven or other appliances can interfere with wireless communications, causing packet fragmentation and data corruption. Another major challenge may arise when multiple radio technologies operate in the same frequency band. Specifically, for example, both IEEE 802.11 (Wi-Fi) networks and 802.15 (Bluetooth) networks operate in the unlicensed 2.4 GHz Industrial Scientific Medical (ISM) frequency band, which can lead to signal interference and result in significant performance degradation when devices are co-located in the same environment.
Sinusoidal interferer signals or frequency tones can be especially problematic. For example, there exist wireless networks that employ a CSMA/CA protocol in which a jam signal comprises a prescribed periodic signal of interest. Stations on such networks transmit jam signals before transmitting data to warn other stations that a data transmission is in progress. Stations on such networks also listen for a jam signal transmitted onto the shared medium by other stations before transmitting their own data onto the medium. Stations detect the jam signal using autocorrelation techniques. If a station detects such a jam signal transmitted by another station, then it delays its own transmission by a time interval sufficient for the other data transmission to complete. Unfortunately, autocorrelation of a sinusoidal interferer signal (or tone) can produce a false detection of a jam signal causing a station to unnecessarily delay data transmission, which can result in reduced data throughput.
Thus, there has been a need to reduce the impact of periodic noise signals such as sinusoidal signals and tones upon the operation of wireless communications devices. The present invention meets this need.